The present invention relates to the processing of images for displaying on a display, and in particular to the processing of images for displaying images on a liquid crystal display.
Video images are displayed on various display devices such as Cathode Ray Tubes (CRTs) and Liquid Crystal Displays (LCDs). Typically such display devices are capable of displaying on a display screen images consisting of a plurality of picture elements (e.g., pixels) which are refreshed at a refresh rate generally greater than 25 Hertz. Such images may be monochromatic, multicolor, full-color, or combinations thereof.
The light of the successive frames which are displayed on the display screen of such a CRT or LCD display device are integrated by the human eye. If the number of displayed frames per second, typically referred to as the frame rate, is sufficiently high an illusion of the images being displayed in a continuous manner and therefore an illusion of motion may be created.
The technique in which images are formed on the display screen of a CRT display is fundamentally different from the way in which images are formed on the display screen of a LCD display. On a CRT display device the luminance of a picture element is produced by an area of a phosphor layer in the display screen where the area is struck by a writing electron beam. On a LCD display device, the luminance of a picture element is determined by the light transmittance state of one or more liquid crystal elements in the display screen of the LCD display device at the location of the picture element, whereby the light itself originates from ambient light or a light source. For accurate reproduction of moving images or moving parts of an image, the luminance response of the used display device is important.
The luminance responses and the luminance response times of CRT and LCD display screens are different. The luminance response time, being the time needed to reach the correct luminance on the display screen in response to an immediate change in a corresponding drive signal, is shorter than a frame period for a CRT display device but up to several frame periods for a typical LCD display device.
For LCD display device, the luminance responses and the luminance response times are different for a darker-to-brighter luminance transition as compared to the responses and response times for a similar brighter-to-darker luminance transition. Further, the luminance responses and luminance response times are temperature dependent, drive voltage range dependent, and, due to production tolerances, unequal over the LCD screen area (location dependent).
One existing technique to change the luminance response times with LCD display devices is to attempt to shorten the overall luminance response times by over-driving all the signals of the display for the slower of the transition of darker-to-brighter and brighter-to-darker. While of some benefit in increasing the temporal response of the display, the resulting image still includes some flickering. Flickering may be observed, in many cases, as apparent flickering of an image as the image is moved around on the display. Flickering tends to be most pronounced when an image is viewed on a shaded background with a dotted pattern as well as vector art often used in computer aided drawings.
Another existing technique to change to luminance response times with LCD display devices is to slow down the transition of all pixels of the display from the darker-to-brighter brighter transition and the brighter-to-darker transition to the slowest transition within the display. This slowing down of the transition may be performed by modification of the driver waveform to achieve the slower temporal response. While slowing down the transition of all the pixels of the display results in a decrease in apparent flicker, unfortunately, the slowing down of the temporal response of the entire display result in objectionable motion blur because of the insufficient effective refresh rate.
EP 0 951 007 B1 disclose a de-flickering technique in which the video signal is modified so that the asymmetry of luminance rise and decay time is compensated. EP0 951 007 B1 is incorporated by reference herein. Referring to FIG. 7, FR which is representative of the present luminance output as it was predicted one frame before (previous frame) is subtracted from the input video signal. This difference and the present luminance output FR are the two inputs to the processing unit. The outputs of the processing unit are ΔC and ΔR, where ΔC is the new correction value to be added to the present predicted luminance FR, and ΔR is the new prediction of luminance change after the next frame.